[Python] Change background after a certain amount of collisions

I am attempting to switch the background only if a certain number of collisions is detected. The collisions are between 7 balloons and a gun. Note that the collision works perfectly fine. Currently, 7 balloons are showed up and each time I hit a balloon it switches. I want to hit 1 balloon out of 7 on the first background, then it changes to the second background where you hit 1 balloon again out of the 7. When you hit a balloon on the second background, it changes back to the first. Then you hit 2 balloons on the first background, then it changes to the second background where you hit 2 balloons again, and so on. How would I do that?

So the number of hits should be like 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7; where the first number in each pair is the number of hits in the first background and the second is the new background.

Here's my code:

import pygame as pg
import random as r
import sys


pg.init()


bg = pg.image.load('bg.jpg')# Background Image #
bg = pg.transform.scale(bg, (688,387))
new_bg = pg.image.load('new_bg.jpg')
new_bg = pg.transform.scale(new_bg, (688,387))

radius = 30
diameter = 2 * radius

num_balloons = 7
bullets_colors_ls = []
iterator = -1

num_balloon_list = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7]
balloon_list_index = 0


def create_balloons():
global balloon_list
global colors

for i in range(num_balloons):
while True:
candidate = r.randint(0, 500)
if all(abs(candidate-x) >= diameter for x in balloon_list):
break
balloon_list.append(candidate)

def draw_balloons(y):
for i in range(num_balloons):
screen.blit(colors, (balloon_list , y-50))


def check_collisions(x, y):
global hit_var
global hit
global score
global scoretext
global bg_bool
global num_balloon_list, balloon_list_index

for i in range(num_balloons):
gun_rect = gun.get_rect(topleft = (x,y))
gun_mask = pg.mask.from_surface(gun)

balloon_rect = colors.get_rect(topleft = (balloon_list, y-100))
balloon_mask = pg.mask.from_surface(colors)

offset = (balloon_rect.x - gun_rect.x), (balloon_rect.y - gun_rect.y)
if gun_mask.overlap(balloon_mask, offset):
hit = True


for balloon_list_index in num_balloon_list:
balloon_list_index += 1
bg_bool = not bg_bool



hit_var = i
score += 2
scoretext = myfont.render("SCORE: "+str(score), 1, (0,0,0))
print(f'hit balloon: {i}')
break










# Vars #
x = 0
y = 250
velocity = 5
score = 0
hit = False
bg_bool = False
testvar1 = True
clock = pg.time.Clock()


screen = pg.display.set_mode((688 ,387)) # Size of the screen #
caption = pg.display.set_caption("Remember") # Title of the window #

balloon_list = []
b1 = pg.image.load('balloons/1.png').convert_alpha()
b1 = pg.transform.scale(b1, (63,131))
b2 = pg.image.load('balloons/2.png').convert_alpha()
b2 = pg.transform.scale(b2, (63,131))
b3 = pg.image.load('balloons/3.png').convert_alpha()
b3 = pg.transform.scale(b3, (63,131))
b4 = pg.image.load('balloons/4.png').convert_alpha()
b4 = pg.transform.scale(b4, (63,131))
b5 = pg.image.load('balloons/5.png').convert_alpha()
b5 = pg.transform.scale(b5, (63,131))
b6 = pg.image.load('balloons/6.png').convert_alpha()
b6 = pg.transform.scale( b6, (63,131))
b7 = pg.image.load('balloons/7.png').convert_alpha()
b7 = pg.transform.scale(b7, (63,131))
colors = [b1, b2, b3, b4, b5, b6, b7]




gun = pg.image.load('game-gun.png').convert_alpha()
gun = pg.transform.scale(gun, (150,150))

create_balloons()



pg.display.flip() # Updating #

running = True # Game loop bool #

while running: # Game loop #
clock.tick(60)

if bg_bool == False:
screen.blit(bg, (0, 0))


elif bg_bool == True:
screen.blit(new_bg, (0,0))
screen.blit(scoretext, (5, 10))



if hit == True:
r.shuffle(balloon_list)
hit = False

for event in pg.event.get():
if event.type == pg.QUIT:
pg.quit()
sys.exit()
if event.type == pg.KEYDOWN:
if event.key == pg.K_ESCAPE:
running = False

if event.key == pg.K_SPACE:
check_collisions(x, y)

draw_balloons(y)

keys = pg.key.get_pressed()
x += keys[pg.K_RIGHT] - keys[pg.K_LEFT] * velocity
x -= keys[pg.K_LEFT] - keys[pg.K_RIGHT] * velocity



screen.blit(gun, (x, y))
pg.display.update()




What changes the background is the bg_bool = not bg_bool and I am iterating through a list of the amount of hits -> num_balloon_list = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7]

You can download all of the images here: Download images on REPL

How can change the background after a certain amount of collisions (hits on the balloons)?

P.S: I have asked a question that is a similar concept with a great answer, but I failed to implement it into this scenario.

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